Recently, interests on energy storage technologies are more increased. As the energy storage technologies are extended to cellular phones, camcorders and notebook PC, and further to electric vehicles, the demand for high-energy concentration of a battery used as a power source of such an electronic device is increased. A lithium ion secondary battery is one of the most satisfactory batteries, and many studies are now in active progress.
A lithium secondary battery developed in the early 1990's includes an anode made of carbon material capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxide, a separator interposed between the anode and the cathode, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a non-aqueous solvent.
The non-aqueous solvent of lithium secondary batteries may use a mixed solvent with polarity in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are suitably mixed. Here, the non-aqueous solvent is controlled to have a low viscosity so as to show good wettability against the separator.
In other words, a lithium secondary battery can be charged or discharged only when the separator is sufficiently wet by the non-aqueous solvent. However, non-aqueous solvents commonly used have a different polarity from common separators such as polyolefin-based porous films or polyester-based non-woven fabrics, so it is needed to improve wettability of the non-aqueous solvent against the separator. If the wettability of the non-aqueous solvent against the separator is deteriorated, charging or discharging of the lithium secondary battery becomes deteriorated or disabled. Accordingly, the wettability of a non-aqueous solvent against a separator is kept over a certain level by using a non-aqueous solvent with a low viscosity or adding a low-viscous organic solvent over a certain amount to lower the viscosity. In common cases, a non-aqueous solvent has a viscosity equal to or slightly more than 1.0 cP at 25° C.
However, using a non-aqueous solvent with a low viscosity may cause water leak, and such a non-aqueous solvent tends to be easily evaporated due to its strong volatile property. In addition, due to strong flammability of the non-aqueous solvent with a low viscosity, safety-related problems such as firing or explosion caused by overcharging, thermal runaway or piercing of the separator become worse. In these days when explosion accidents of lithium secondary batteries come to social issues, such safety-related problems are considered more seriously.
Thus, there is still a strong demand of using a non-aqueous solvent having a high viscosity (commonly a viscosity of 1.4 cP or above at 25° C.) with a high flashing point. However, if a non-aqueous electrolytic solution containing a non-aqueous solvent with a high viscosity of 1.4 cP or above at 25° C. is applied to common separators such as polyolefin-based porous films or non-woven fabrics, the wettability against the separator becomes poor, so the charging or discharging performance of the lithium secondary battery is greatly deteriorated or disabled.
To solve this problem, there have been proposed many methods, such as adding a surfactant or applying heat or pressure when a non-aqueous electrolytic solution is injected, but these methods need additional processes and thus they are not economical.
Meanwhile, Korean laid-open patent publication No. 2007-0019958 discloses a separator having a porous coating layer formed by coating at least one surface of a porous substrate having a plurality of pores with a mixture of inorganic particles and a binder polymer. A separator of such a structure prevents a short between the anode and the cathode when the porous film is thermally shrunken or the separator is pierced, thereby enhancing safety of the lithium secondary battery. However, though a lithium secondary battery has such a separator, the above problems caused by a low-viscous non-aqueous solvent are still not solved.